![]() A wide variety of proprietary pastes and backing materials are available than can be use to protect the root instead of a gas shield. If good corrosion resistance of the root is required, the oxygen level in the dam should not exceed 0.1% (1000 ppm) for extreme corrosion resistance this shouldīacking gasses are typically argon or helium nitrogen is often used as an economic alternative where corrosion resistance is not critical, nitrogen + 10% helium Once purging is complete, the purge flow rate should be reduced so that it only exerts a small positive pressure, sufficient to exclude air. Welding should not commence until sufficient time has elapsed to allow the volume of purging gas flowing through the dam to equal at least the 6 times the volumeĬontained in the dam. The gas shield should be contained around the root of the weld by a suitable dam, which must permit a continuous gas flow through the area. To ensure good corrosion resistance of the weld root, it must be protected from the atmosphere by an inert gas shield during welding and subsequent cooling. It is also called weld decay since it usually happens during welding process when the zone around the weldįigure 2 shows migration of chromium during heating of stainless steels.įigure 2: Migration of chromium during heating of stainless steels. ![]() Grains resulting in decreasing the corrosion protective passive film. This causes depletion of chromium from the austenitic If any part of stainless-steel is heated in the range 900-1400☏ (482-760☌) for any reasonable time there is a risk that the chrome will form chromeĬarbides Cr 23C 6 with any carbon present in the steel along the austenite grains. Therefore it is recommended that stainless steel fabrication be carried out in a separate designated area and special stainless steel (passivation reduces the anodic reaction involved in the corrosion process).Ĭarbon steel tools, also supports or even sparks from grinding carbon steel, can embed fragments into the surface of the stainless steel. Once cleaned, the surface can be chemically passivated to enhance corrosion resistance, (acid pickle with a mixture of nitric and hydrofluoric acid). Layer must be removed, either mechanically (grinding with a fine grit is recommended, wire brushing and shot blasting will have less effect), or chemically Oxide layer, distinguished by its blue tint, will have a chrome-depleted layer under it, which will impair corrosion resistance. Is not adequately protected from the atmosphere during welding or is subject to very heavy grinding operations, a very thick oxide layer will form. This film reforms rapidly by reaction with the atmosphere if damaged. Stainless steel has a very thin and stable oxide film rich in chrome. Figure 1 shows the intergranular corrosion. If the precipitates at the grain boundaries have higher electrode potential the grains will dissolve (anodic reaction). The process results in deterioration of the bonding between the grains and drop of mechanical properties. Dissolution of anodic grain boundaries starts from the surface and advances along the Value of electrode potential will provide cathodic reaction (reduction). If the phases segregated at the grain boundaries have lower value of electrode potential they will oxidize (anodic reaction) and the grain metal having higher Is the difference between the electrode potentials of the grain boundary and the grain itself, which form a galvanic cell in presence of an electrolyte. The driving force of intergranular corrosion Intergranular corrosion is caused by microsegregation of impurities and alloying elements on the grain boundaries. This form of corrosion results in a loss of strength in metal parts where the grains Have difficulty in detecting the early stages of intergranular corrosion. Some causes of intergranular corrosion are welding, stress annealing, improper heat treating or overheating in service. Intergranular corrosion, also called intercrystalline corrosion, occurs on or adjacent to the grainīoundaries of a metal. Grains are small crystals whose surfaces join the surfaces of other grains to form The microstructure of metals and alloys consists of a granular composition. Intergranular corrosion of stainless steels
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